Yamaha TRX850 Enthusiasts Forum

I've had my TRX in NZ in the shop getting checked out as after sitting for nine months while I've been in working/living in Aussie it was blowing like a two stroke out the the crank case breather, it was only firing up on one cylinder and then taking on the second after 30secs of warming up. I did a quick compression test at home with my mates cheap compression tester and I got 155psi on on the 'good' cylinder and 50psi on the dud cylinder. The shop has done a leak down test and revealed 75psi on the dud cylinder and 225psi on the good one.

Does anyone know of a piston kit that would produce such high compression results?

I was under the impression that my bike had stock pistons with a cam (never had it open to check), and was intending to go to an 878cc kit if it looked like the cylinders were going to need work... now I've asked the mechanic to strip the head off and send me a photo of the pistons to see what we're dealing with as obviously 225 PSI is quite up there.

I would be suspecting the mechanics comp gauges need a look at,if 155 is standard, would you not be having pre detonation problems at that kind of compression?

A leak down test is a completely different thing to a compression test. Compressed air is admitted into the cylinder and the amount of leakage is measured (as the name suggests). You cannot use the figures of one for the other. Did you do the compression test correctly (engine at operating temp and throttle wide open, crank until reading peaks).

Sorry, should say they did a compression test.

I'm guessing the maths is wrong, but if his reading is correct it works out to a compression ratio of around 13.5:1 which is supersport race motor territory.